Electrochemical cells, especially high energy density cells such as those in which lithium is an active material, are subject to leakage or rupture by various abusive treatment, which, in turn, can cause damage to the device which is powered by the cell or to the surrounding environment. In the case of rechargeable cells self-heating occurs as such cells are charged. Charging at too rapid a rate or overcharging can lead to an excessive increase in temperature. When the temperature exceeds a certain point, which varies depending upon the chemistry and structure of the cell, an undesirable and uncontrollable thermal runaway condition begins. In addition, because of the overheating and/or chemical reaction, internal pressure builds up, and electrolyte may suddenly be expelled from the cell.
Conventional cell designs employ an end cap fitting which is inserted into an open ended cylindrical casing after the cell's active material, appropriate separator material and electrolyte have been inserted therein. The end cap is in electrical contact with one of the anode or cathode material and the exposed portion of the cap forms one of the cell terminals. A portion of the cell casing forms the other terminal. Typically, various loose components are received in the end cap to provide over-pressure, short circuit and/or over-temperature protection. The typical header includes one or more seals to prevent leakage of the electrolyte through the header. However, these seals tend to leak over time due to temperature and pressure conditions and the like. Conventional cells frequently employ polymer PTC (positive temperature coefficient of resistivity) components for over-current protection which are susceptible to deformation when the header is crimp sealed to the electrochemical cell during manufacture. Deformation of the PTC component adversely effects its performance. Further, polymer PTC inherently results in relatively low ampere capacity and high resistance.
Copending U.S. Pat. No. (Ser. No. 08/720,585- A40743), which is incorporated herein by this reference, shows a current interrupt device responsive to over-temperature and over-pressure conditions which is incorporated into an end cap assembly which, in turn, is welded into the open end of the casing of an electrochemical cell to form a hermetic seal. This end cap assembly has particular application to rechargeable (secondary) cells, such as lithium-ion, nickel metal hydride, nickel cadmium and the like, to overcome the danger of the cell over-heating and pressure building up in the cell during exposure to high temperatures, excessive or improper charging, or shorting of the cell. While the current interrupt device of the patent is very effective, it would be desirable to provide an end cap assembly which has fewer component parts and which can be crimped into the casing of the electrochemical cell to lower the cost of the device while forming an effective hermetic seal. Additionally, it would be desirable to integrate an additional safety feature relating to low rate overcharge and overdischarge protection.